Technical Field
The present disclosure relates to a circuit and method for protecting data stored in a memory, and in particular to a circuit and method of data protection for memories having built-in self-test (BIST) circuits.
Description of the Related Art
Random access memories such as SRAMs (static random access memory) and DRAMs (dynamic RAM) generally comprise testing circuits, such as BIST (built in self test) circuits, allowing testing of the memory cells forming the arrays. For example, the test sequences made available by the BIST circuits may include sequences in which test data is written to and then read from certain portions of the memory array.
For some applications, memories may be employed to store sensitive data, which should not be accessible to unauthorized devices. For example, the sensitive data may include cryptographic keys, passwords, or financial or medical data. The BIST circuit of a memory may provide an entry point for an attacker to gain access to the sensitive data. Indeed, the BIST circuit will generally allow test sequences to be performed, and among the available test sequences, some may allow the contents of the memory to be read out, for example as a memory dump. Therefore, to protect sensitive data, the BIST circuit may be partially or entirely deactivated after testing has been completed at the end of the manufacturing process, such that the contents of the memory can no longer be read using a BIST test sequence. For example, BIST circuits may be deactivated using a one-time programmable fuse.
However, for some memories, it may be desirable to permit testing during their lifetime. For example, if during its lifetime a memory starts to malfunction, testing can be used to identify the source of the malfunction. Such a diagnosis may permit the memory to be repaired, or the circuit design to be improved for future products.
There is thus a need in the art for a solution permitting test functions to be applied to a memory without allowing sensitive data to be obtained by unauthorized parties.
The subject matter discussed in the Background section is not necessarily prior art and should not be assumed to be prior art merely as a result of its discussion in the Background section. Along these lines, the recognition of one or more problems in the prior art discussed in the Background section and the subject matter associated therewith should not be treated as prior art unless expressly stated to be prior art. Instead, the discussion in the Background section encompassing one or more recognized problems in the prior art should be treated as part of the inventor's approach to the particular problem, which in and of itself may also be inventive.